Once upon a time, it meant something to be happy and gay. I guess it still does, but not the same thing.
Once upon a time, making love to somebody involved no physical contact. I guess that’s gotten better!
Once upon a time, vanilla was an exciting and exotic flavor.
What happened?
Words change as they get used. People put them together in new ways and make new meanings out of them. Sometimes to just be creative, sometimes to have a euphemism they can say somewhere they weren’t allowed to say before. Sometimes those new meanings stick. When that happens, sometimes the old meaning has to go.
Sometimes when a word is used properly, it still can end up regulated to only one meaning, because of the strength of association with that meaning.
Once upon a time, strangers would stop to have intercourse in the street.
Once upon a time, people ejaculated in anger.
A few examples of phrases we don’t hear much anymore, or if we do, they often mean something new to our ears: kick against the pricks, eager beaver, bull session, get your dander up, stick your oar in someone else’s boat, to get held over a barrel, get your comeuppance. You get the idea.
Many of our old idioms get lost in time this way. We stop using them because people no longer understand the meaning, or the meaning becomes a crude joke.
A great place to poke around if you like this sort of thing is TV Tropes.org. If you do like this sort of thing, be prepared to spend a long time on their site!